The present invention relates to an RF signal processing circuit for processing RF signals from the tracks of an optical disk.
As used herein, and as widely employed and understood within the relevant art, the term "RF signal" indicates the electrical output signal from an optical pickup or the like which senses recorded patterns on an optical disk.
FIG. 1 shows recorded patterns on the tracks of an optical disk. Each sector of the optical disk includes plural servo blocks (e.g., 43 servo blocks). The sector includes a preformat portion composed of a servo-byte of two bytes and a recording portion succeeding thereto composed of a data-byte of 16 bytes. The servo-byte includes a clock pit and two wobbled pits located to the right and left with respect to the center of the track.
When a pickup (that is, a beam spot for detecting information) traces the center of the tracks with no deviation, no difference occurs in the light intensities reflected from the two wobbled pits. On the other hand, when the pickup deviates to the right or left from the center of the track, a difference corresponding to the direction and distance of the deviation occurs between the light intensities reflected from the wobbled pits. Accordingly, a tracking error signal can be produced by detecting the difference between the reflected light intensities from the two positions (two wobbled pits), that is, by detecting the level difference between the RF signals from the two wobbled pits. The tracking error signal thus produced is held for the period during which a databyte succeeding the servo-byte is read out. The respective pairs of wobbled pits are arranged in such a manner that longer and shorter intervals thereof are alternately repeated every 16 tracks so that, by detecting variations in these intervals, the number of tracks can be accurately counted (16 tracks), even if the pickup searches the tracks at high speed.
The distance D between the clock pit and the one of the two wobbled pits closer to the clock pit is set to a length which does not exist in the data-byte. Therefore, the distance D is useful for producing a synchronizing signal. For example, a pre-pit portion including a first pit portion for tracking and a second pit portion for producing a synchronizing signal can be provided in the servo-byte area. The second pit portion contains a first byte of an eight-channel pit and second byte of a 12-channel pit, the distance therebetween being set to a 19-channel clock. The data-byte area is less than an 18-channel clock in length, and therefore the second pit portion having the interval of the 19-channel clock can be detected as a synchronizing signal.
Various timing signals are produced on the basis of the synchronizing signal, and a clock signal is generated in response to a detection signal of the clock pit.
The mirror-surface portion having the length of D is called the focusing area, and is used for producing a focus error signal which is held for a period when information from the succeeding data-byte is read out.
Such pits as described above are formed in each servo-byte area. Data required by the user are recorded in a data-byte area (recording area) succeeding the servobyte area with pit patterns or with phase-variations or the like.
In the case where the data are recorded in the form of a pit pattern, the output RF signal from the pickup drops in level in positions where pits are formed. On the other hand, in the case where the data are recorded using a phase-variation technique, the level of the RF signal is increased in recorded positions corresponding to the positions where pits are formed for some recording materials. For recording or reproducing data in both types of disks described above, a DC amplifier having a broad bandwidth must be used to amplify the RF signal because the potential of the servo-byte area is unstabilized.
However, such a DC amplifier of broad bandwidth has disadvantages in that its noise level is inherently high, drift variations with respect to changes of temperature easily occur, and the cost is high.